The performance of electronic devices is strongly related to the thermal properties of materials used in their fabrication. In particular, for high power devices, the more effectively the excess heat they produce is removed, the lower their operating temperature, the greater their efficiency, and the longer their lifetime. With devices becoming smaller and smaller and power densities rising, the management of the thermal aspects of device operation becomes increasingly important.
One class of semiconductor devices for which this invention is particularly suitable are those which use gallium nitride as the semiconducting component. Gallium nitride (GaN) is a wide band-gap semiconducting material with electronic properties that make it well suited to high power, high brightness, light emitting diodes (LEDs) and high power, high frequency (1-100 GHz) devices, for example p-High Electron Mobility Transistors (HEMTs). Currently available GaN devices use hexagonal gallium nitride monocrystalline layers grown epitaxially onto a variety of monocrystalline substrates, including: hexagonal sapphire, hexagonal silicon carbide and {111} silicon. However, there are disadvantages with each of these. The typical configuration is a wafer of the substrate material, for example 300 μm thick, onto which is grown a buffer layer of AlGaN with a thickness of the order of 1 μm or so, where the concentration of aluminium is varied to control the lattice parameter and reduce the interface strain, and then finally a layer of device quality GaN, which may typically have a thickness in the region of 10-50 nm, is grown on top of the buffer layer.
The substrate onto which GaN devices are grown is required to act as a heat spreader (but commonly referred to a heat sink), that is to initially spread and then remove the heat from the “hot-spots” within the GaN devices (generally generated at the junctions). Sapphire and silicon are readily available and relatively cheap, so sapphire tends to be used as the substrate for LEDs (where the light comes through the substrate, i.e. the substrate is required to be transparent in the visible spectrum), while silicon is used as a substrate for the high power Radio Frequency (RF) devices. (Silicon has a higher thermal conductivity than sapphire and so is better in applications where more heat is generated). However, both silicon and sapphire limit the power output of GaN devices grown on them because neither silicon nor sapphire have a very high thermal conductivity (<150 W/m·K and <45 W/m·K respectively). The thermal conductivity of GaN is itself not that high, typically about 130 W/m·K. Single crystal hexagonal silicon carbide (typically the 4 H or 6 H polytype) is both transparent and of high thermal conductivity (>500 W/m·K) so it would be the preferred substrate for all GaN devices if it were readily available in both large diameters and at low cost. However, this is not the case.
It is well known that CVD diamond can be grown onto both polycrystalline and monocrystalline silicon substrates, and that CVD diamond can have a thermal conductivity in excess of 1000 W/m·K, with the best material having a thermal conductivity in excess of 1800 W/m·K. It is less well known that CVD diamond and silicon are reasonably well matched in terms of thermal expansion coefficient, which allows thick diamond layers to be grown onto silicon surfaces.